1. Field of Invention
The present invention relates to the extrusion of a tire tread with a thin electrically conductive strip of material for discharging the electro static charges produced by a vehicle having tires with non-conducting rubber treads.
2. Description of the Related Art
In European patent application EP 0 925 903 A1, it is proposed to provide a narrow high conductive rubber strip at a widthwise center region of the tread. This is accomplished by adding a detachable sub insert block to the extrusion head insert block. In order to provide the precise installation of the sub insert block substantial machining and revision of the insert block has been required. Also the sub insert blocks were fabricated of sheet metal to provide a thin wall sheath portion and a base portion which is received in a cutout in the insert block. The sub insert blocks also had walls, which were tapered to direct flow of the high conductive rubber.
U.S. Pat. No. 5,397,616 is directed to a method of providing unvulcanized tread material at the bottom portions of tire grooves in the tire tread by extruding the tread with projecting portions of the base layer adjacent portions of the cap layer having grooves whereby the base layer is exposed at the groove bottoms in the manufactured tire. This exposure of the base layer is dependent upon the depth of the grooves and penetration of the cap layer and being recessed does not provide contact with the roadway as is required for discharging static electricity during operation of a vehicle.
With the present invention a narrow strip of conductive rubber is formed by conveying the conductive rubber a lower the undertread flow channel in a tire tread die through a passage into a chimney block positioned in the flow channel for the non-conductive tread rubber and through a slot in the chimney block. Not only is the narrow strip in position for discharging static electricity generated by the vehicle but the face of the strip at the tread surface is not smeared.
In accordance with one aspect of the invention there is provided a tire tread die comprising an outer tread passage for forming an electronically non-conductive rubber outer tread, a lower undertread passage for forming a lower undertread layer of electrically conductive rubber characterized by:
(a) a chimney block mounted in the outer tread passage and extending from an inner surface to an outer surface of the passage;
(b) a chimney block opening extending from one end of the chimney block to an opposite end;
(c) an undertread orifice in the outer tread passage in communication with the lower undertread passage and with the chimney block opening; and,
(d) a slot in a downstream side of the chimney block in communication with the chimney block opening and extending between the one end and the opposite end of the chimney block for communicating a narrow strip of the lower undertread layer to the outer tread in the outer tread passage;
(e) a final die downstream of the outer tread passage and the lower undertread passage for receiving the lower undertread layer and the outer tread with the narrow strip of lower undertread layer extending from the undertread to an outer surface of the outer tread.
In accordance with another aspect of the invention, there is provided a method of forming a tire tread with a lower undertread layer of electrically conductive rubber and an upper tread having a narrow strip of electrically conductive undertread rubber extending from the lower undertread layer to the surface of the upper tread comprising:
(a) conveying the electrically conductive rubber of the lower undertread layer through a first passage of a tire tread die;
(b) conveying electrically nonconductive rubber of the outer tread through a second passage of the tire tread die;
(c) diverting a portion of the electrically conductive rubber of the lower undertread layer from the first passage to the second passage through an undertread opening in the first passage;
(d) conveying the electrically conductive rubber of the lower undertread layer from the undertread opening to a position extending across the second passage in an opening of a chimney block extending between opposite surfaces of the second passage;
(e) conveying the electrically conductive rubber of the lower undertread layer from the second passage through a slot in the chimney block opening to form the narrow strip in the upper tread and;
(f) conveying the lower undertread layer and the upper tread with the narrow strip into a final die having an opening in communication with the first passage and the second passage to position the upper tread with the narrow strip over the lower undertread layer and provide electrical conductivity of the tread.
In accordance with another aspect of the present invention, the chimney block may be welded to an upper plate of the outer tread passage to provide the narrow strip of the lower layer in the outer tread without smearing.
In accordance with another aspect of the present invention, the protrusion may have a flow splitting tapered end at the upstream side and tapered walls from the upstream side to the downstream side of the chimney block to divert the flow of the outer tread around the chimney block.
Still other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.